PAPERS

The Reproduction of Sound in Auditory Perspective and a Compatible System of Stereophony*

FR. HEEGAARD

Statsradiofonien of Denmark

A little more than a year ago we invited the late Holger Lauridsen to contribute to the E.B.U. Review an article describing his work on reproduction and in particular the system that he had proposed for stereophonic broadcasting without most of the disadvantages of using separate transmitter chains for the left-ear and right- ear components. Many of our readers will recall Mr. Lauridsen's untimely death in December 1957, which put an end to his valuable contribution to the art and science of broadcasting. Recently, however, interest in the possibilities of stereophony, and particularly in stereophonic broadcasting, has become widespread, and Mr. Heegaard very kindly agreed to write the following article, which is based upon the work of Mr. Lauridsen. Editor

LAURIDSEN'S M-S TECHNOLOGY by Blumlein insum-differencesignalformats, as found in the references. The experimentsof Holger Lauridsenat DanishState DUANEH. COOPER Radio, leading to demonstations in Copenhagen in Chaml_aign,IL 61820, USA 1954June, dealt with the perceptionof space, evoked in allocating two channels, a main channel (M) to direct sound and a second (S) to reverberant sound --to be reproduced with either or * This paper is reprinted from E.B.U. Review, Part loudspeakemmand the qualities of the sound-field A_Technical, no. 52, pp. 2-6, 1958 December, illusion that depend on insertion and polarity published by the Technical Centre of the European inversion. Reproduction of spatial sound by M-S Broadcasting Union. array--a transverse dipole radiator (S) Reprinting here is intended to honor the seminal work in coincidence with a monopole radiator (M)--was of Holger Lauridsen in exploring spatial perspective shown to be an alternative to reproduction by spaced in sound reproduction and in calling attention to sum channels, left as M + S and right as M - S. In the -difference, or M-S, matrixing in his own research reproduction of images, by contrast, stereophonic and in the (at the time widely neglected) earlier work qualities and lateral spread were observed in the re- of . The term "MS" originates with productions of sound from the M-S Lauridsen, surely one of the second-wave, if Blumlein array--a cardioid directed inmidplane at center stage is one of the first, stereo pioneers. Lauridsen's original (M) and a dipole rotated 90° for side (S) pickup.-The M-S microphone, as shown in Fig. 4 of this paper, mnemonics mid-side or mono-stereo are suggested, remains on exhibit at the Danish Radio Museum in with left and right (or twin-channel A, B) signals con- Copenhagen. Lauridsen's own writings are in Danish structed by sum and difference. Compatible M-S and German, for which translations to English, unfor- stereo relies on forming M as A + B and S as A - tunately, are not readily available. We are indebted to B in applications for which M and S are suited as the Danish Radio Corporation for their cooperation, transmission, FM broadcast, or recording channels, particularly to Dan Popescu for his assistance and to Problems with phase incoherence for spaced micro- Peter Hansen, Direction of Engineering (Retired), who phones are noted, along with mention of prior work prepared the biographies published with this paper.

802 J. AudioEng.Soc.,Vol.40,No.10.1992October PAPERS AUDITORY PERSPECTIVE AND COMPATIBLE STEREOPHONY

INTRODUCTION impression of an added dimension to the imaginary room in which the music seems to take place. The There seems to be little doubt that any major im- resemblance of this arrangement to that of Vermeulen provement in the quality of broadcast sound reproduc- and Kleis at Eindhoven will be recognized [5], [6]. tion must be towards the creation around the listener Feedback may be applied to the delaying machine of a sound field having a spatial distribution approxi- in order to simulate secondary reflections, thus ap- mating much more closely to that in the broadcasting proximating more closely to the reverberation process studio than is the case with ordinary "single-channel" in a real room. Although this does add some naturalness broadcasting. It may even be said that the improvements to the effect, it is surprising to note the relatively small brought about by the introduction of VHF broadcasting, importance of the amount of feedback used. It seems leading to a better reproduction of the higher audio to be the first few reflections that create the impression frequencies, have not always led to happy results, be- of a large room. cause the radiation of strong components at those fre- The experiment is rather instructive, because it dem- quencies from a concentrated source has often been onstrates the fact that naturalness of reproduction may found to Cause a certain irritation, especially when be greatly increased without establishing any possibility broadcasting from extensive sources, such as an or- of right and left localisation. When listening to a re- chestra. It is true that the introduction of spherical cording of organ music it is quite easy to imagine that may to some extent alleviate this effect, one is present in a large church. but even this does not cure the trouble completely, It is perhaps not very surprising that the arrangement because the loudspeaker remains essentially a point described creates such an impression, seeing that the · source which is easily located by the listener, sound from the loudspeakers approximates to what Certain experiments which were carried out in the would be obtained in a large room. It is rather less laboratories of the Danish State Radio and which shed easy to imagine exactly by what mechanism of hearing some light on the problems of spatial sound reproduction the psychological effect is obtained. Since, however, will be described in this article. These experiments led the effect must be obtained solely as a result of the to a compatible system of stereophony which was dcm- sound pressure at each ear as a function of time, one onstrated for the first time at a meeting in "Nordisk might well ask whether the use of spaced loudspeakers Akustisk Selskab" in Copenhagen on 6th June, 1954 is really necessary. Could not similar spatial effects be [1], [2]. obtainedwhen listening with a pair of headphones? The answer was found to be yes, and the method of SOME EXPERIMENTS PRODUCING A achieving these effects is both simple and surprising. SUBJECTIVE EFFECT OF SPACE Consider Fig. 1. We have a pair of headphones to which is delivered a signal from a musical recording. The first experiment is very simple: A loudspeaker The headphones are connected so that the diaphragms is placed in front of an observer and another behind move in phase; by this is meant that, at any instant, his back. Musical signals are fed to the loudspeakers, they both move towards the head or they both move The effect of the back loudspeaker is to some extent away from it. A delayed signal from the same source to lessen the localisation of the front loudspeaker as a may also be fed to the headphones through a separate sound source. The distances between the observer and circuit, and in the leads to one of the phones in the the two loudspeakers are rather critical and the effect delayed circuit is connected a reversing switch. The is not very marked although a certain sense of roominess delayed signal may thus be led to the headphones in is achieved, phase or in antiphaseat will. The delay is again of the If, however, the sound from the back loudspeaker order of 50-100 milliseconds. is delayed by means of a by some 50- We start the experiment by listening to the primary 100 milliseconds, the effect is completely different. The observer gets the impression of being present in a large room in which music is played in front of

him. The loudspeaker distances are no longer critical, recordmgv.,,,,. because localisation of the back loudspeaker is sup-

pressed by the Haas effect. This is a modern version _ [-5-1 '1 1926, which consisted of a gramophone with two sound- I boxes placed one behind the other on the record and eachleadingto itsownhorn. [-5-3 of Thethe "Ultraphon"arrangement demonstratedmay be improvedby Kiichenmeisterby the additionin [ _ ___U.,jp_......

the observer. The lateral loudspeakers are fed in parallel of a loudspeaker to the left and another to the right of _ ] [_Z3 0'_ fSo° with delays of some 50-100 milliseconds, the delay {-5'-3 of the back loudspeaker being then 100-200 milli- seconds. This arrangement gives the observer the Fig. 1. Circuit for reproducing sound with an effect of space.

J. Audio Eng. Soc., Vol. 40, No. 10, 1992 October 803 HEEGAARD PAPERS signal only. The effect is the usual one when listening shot seems to start in the back of the head and fly away. with headphones. The sound seems to come from no- Another loudspeaker arrangement has also been used where in particular or seems perhaps to be localised in for these experiments. This is shown in Fig. 3. One the head itself. We add some delayed sound, first with loudspeaker is placed in the usual fashion in front of the headphones connected in phase. The effect is that the observer and a second, which we shall call the of added reverberation and, due to the rather long delay, transverse loudspeaker, is placed behind the first and we soon arrive at a point in which the clarity of repro- with its edge towards the observer. When primary duction is impaired by. too much reverberation. But sounds are fed to the front loudspeaker and delayed the feeling, or rather lack of feeling, of localisation is sounds to the transverse loudspeaker, we get all the not changed. We then operate'the reversing switch, so effects described above. If this arrangement is placed that the delayed sound is delivered in antiphase. We in an ordinary room the reflections from the side walls immediately get the impression of being present in a reinforce the output from the transverse loudspeaker. large room in which the music is being played, the The arrangement also works, however, in the echo- distances to the walls corresponding to the delay time. free room, although the sound from the transverse For some reason, the sound field seems to be confined loudspeaker decreases rather rapidly with distance, es- to the space behind us, as if we were placed with the pecially at low frequencies, due to the cancellation of face against a wall. The music regains its clarity and the sound pressure from the two sides. This gives rise the effect is very pleasing. It is also very convincing to a remarkable effect: if we wall( towards the loud- -in many cases where the experiment has been dcm- speakers while music is being played, the distance to onstrated without previous warning, the observers have the sound source seems to increase. torn off the headphones and looked behind in order to find the soundsource. STEREOPHONIC REPRODUCTION The experiment may also be made with two loud- speakers instead of headphones. In order to avoid con- In the experiments so far described, the effects have fusion due to reflections from the walls of the room in been completely symmetrical in regard to left and right. which the experiment is made, it is best to use an ane- No stereophonic effects have been involved. A little choic chamber such as is used for microphone testing, consideration will show, however, that it must be pos- The loudspeakers may be placed to the right and left sible to obtain stereophony from the reproducing ar- at one end of the room in the position usually employed rangements shown. Consider for instance Fig. 2. Mu- fo...... -,_h,mn_, stereophonic reproduction. _nc toua- sicai signals are fed to the terminals marked M. The speakers may be fed through a bridge circuit (Fig. 2). two loudspeakers operate in phase, and the sound source The effect is no less striking than with headphones, seems therefore to be located at the midpoint between With this arrangement another curious effect may be the loudspeakers. The same signals are now led to the demonstrated. For this we use a recording of a pistol terminals S, without introducing any delay. The two shot, fired in an echo-free room. This is reproduced sets of signals will add in phase in one of the loud- with the primary sound in phase, as before, but instead speakers, say A, and in antiphase in the other. The of adding one delayed sound pulse we add two, one sound source seems to have moved to the left. If we after the other and both in antiphase (terminals S). The gradually attenuate the S signals to zero, reverse the shot seems to start from a point far away in a direction connections and increase the signals again the sound exactly in the middle between the loudspeakers and to source seems to be moving from left to right. What is fly towards the observer, ending up in the back of his needed to make up a complete stereophonic system is head! If the connections are interchanged so that the therefore a microphone combination which will deliver primary sound is delivered in antiphase and the delayed a) an M signal containing the central information-- sounds in phase, the direction of travel is reversed: the this might be any ordinary microphone--, and b) an S signal which will be zero for a centrally located sound source and increase with increasing angular deviation,

/ the phase reversing as the source passes from left to

_'_ characteristic of which is a figure-of-eight, placed so that the plane of zero sensitivity coincides with our · centralplane. _ The microphonecombinationactuallyused in these M _ ' _ ' experiments consists of two capacity of _]/t B/_/ _ right. For this we may use a microphone, the directional '4 microphone being in the cardioid condition while that the type with steerablepolar diagrams, that of the M S (see Fig. 4). Care should be taken to place the micro- ofphonesthe Sasmicrophoneclose togetheris inasthepossiblefigure-of-eightin order toconditionpreserve phase relationship, which is especially important at Fig. 2. Spatial effect obtained with two loudspeakers, low frequencies. The results obtained in practice are

804 J.AudioEng.Soc.Vol., 40,No.10,1992October PAPERS AUDITORY PERSPECTIVE AND COMPATIBLE STEREOPHONY

[] $l 3.4 Fig. 3. Stereophonic reproduction with loudspeakers arranged perpendicularly. M--mid-plane signal; S--side signal. good; the sideways location leaves little to be desired, sound sources. The letter S might similarly be taken the feeling of depth is retained and the "hole in the to stand for "stereophonic" since the S channel contains middle" effect seems to be less evident than with most the supplementary information necessary to convert other twin-channel systems, the signal to a stereophonic. It could also In traditional twin-channel systems, the information stand for "side" since it defines the side position of the intended for the left ear is sent out in one channel, say sound source. channel A, while the information intended for the right ear is transmitted in channel B. The system here de- COMPATIBLE STEREOPHONY scribed is certainly a twin-channel system, but the di- vision of information between the channels is quite In a sound broadcasting system, it would therefore different. The letter M may be taken to stand for "men- be possible to arrange for the M channel to contain the aural," since the M channel contains all the information signal necessary to operate the usual monaural receivers. necessary for monaural reproduction. It could also stand The addition of an S signal in another channel would for "middle" since the M microphone would usually make stereophonic reception possible for those who he directed towards the acoustical centre of extended had acquired the necessary additional receiving equip-

&

&

1

Fig. 4. Microphone arrangement for M-S stereophony.

J. Audio Eng. Soc., Vol. 40, No. 10, 1992 October 805 HEEGAARD PAPERS

ment, without in any way disturbing the reception of system of stereophonic gramophone recording and even the monaural receivers. The system is therefor corn- used the combination of an omni-directional and a ve- patible with existing sound broadcasting systems in locity microphone to derive directly what have here the same sense as the NTSC system of colour television been called M and S signals [1 1], [12]. Judging from is compatible with the F.C.C. monochrome television contemporary technical literature, these inventions do system. It should for instance be quite possible to mod- not seem to have created much interest, and no one ulate the S signal on a sub-carrier in existing FM stations seems to have conceived the idea of broadcasting pure without interfering with transmission of the M signal. M and S signals in compatible form, or to have realised Other ways of transmitting the S signal could undoubt- the potentialities of such a system. edly be devised. If the operationsM + S andM - S are carried out If stereophonic broadcasting were to take place on on the polar diagrams of Fig. 4, it will be found that the A-B system there would seem to be two alternative the A and B signals derived from the corresponding methods of dealing with monaural receivers, either to microphone combination are the same as those that let them receive the A or B signal as best they might, would be obtained from two supercardioid microphones a procedure which would hardly satisfy the descrimi- placed together with their axes at an angle [7]. The nating listener, or to send out the central, monaural practical results obtained with the system here described signal in a third channel, which for obvious reasons will therefore not be substantially different from those would be out of the question, obtained with the Stereosonic system. In Fig. 2 the signals fed to the loudspeakers are in Other reproducing arrangements than that of Fig. 2 the usual A and B form. The relation between the two are possible, such as that of Fig. 3_ The close similarity forms is simply A = M -4-S and B = M - S, and the of this with the microphone arrangement of Fig. 4 will circuit of Fig. 2 may also be used to convert the A-B be noted, and it was in fact this scheme that was first form to M-S. tried. Whenlistening to this in an anechoic chamber, The idea of transmitting A + B and A - B in order one is surprised to find virtual sound sources in direc- to achieve compatible stereophony is not new [12] al- tions far beyond the loudspeaker"base." A disadvantage though this was not known to Mr. Lauridsen when he of the arrangement is that the S sound decreases rapidly was working on his system. It should be noted, however, with distance; an advantage is that phase relationship that A + B cannot be depended upon to form a satis- of the S signal is inherently preserved. factory M signal when spaced microphones are used A short loudspeaker column with four reproducers to derive the A and B signals. With a combined mi- in a row has also been used. The loudspeakers are crophone such as that used in the Stereosonic system connected in a bridge circuit so that 211four are in the conversion is completely satisfactory [10]. phase for M signals while the two end groups are in Subsequent studies of the patent literature have indeed antiphase for S signals [4]. Here again the stereophonic revealed the fact that more than a quarter of a century effect does not extend very far from the loudspeakers, ago Blumlein used sum and difference signals in his but a very good effect is obtained at a short distance.

1

Fig. 5. Stereophonic reproduction using a group of four loudspeakers in a line. All four are fed in phase with M signals, while the inner and outer pairs are fed in antiphase with S signals.

806 J. Audio Eng. Soc., VoL 40, No. 10, 1992 October PAPERS AUDITORY PERSPECTIVE AND COMPATIBLE STEREOPHONY

Walking into the "active" zone gives the effect of going to get the same result. By adding the two into the middle of a concert hall; the apparent volume velocity outputs and substracting this from the pressure increases, and the arrangement might prove useful in output, it is possible to derive a voltage which is a cases where one or two people wish to enjoy a concert function of the vertical component. If this is applied without disturbing the neighbours, to a separate oscilloscope beam, it is possible to de- termine the direction to the sound source in three di- BROADCASTING STEREOPHONY mensions. By using an impulsive sound source the goniometer The introduction of stereophonics into sound broad- is well adapted to the determination of the directions casting would of course bring many problems in its of reflections in a room. The oscilloscope beam may train. One is connected with the application of multiple be modulated with suitable gating pulses, so that it is microphone techniques. It has generally been considered possible to determine the direction and magnitude of that, in stereophonics, it is preferable to rely on a single reflected pulses in any predetermined time interval. pair of microphones in order not to spoil the directional effect. This would be quite satisfactory when broad- Bibliography casting for instance a classical symphony, where the single microphone technique has already become almost [1] H. Lauridsen, "Nogle Fors6g med forskellige standard. In other fields however such as light or dance Former for rumakustisk Gengivelse" (Some experiments music, this would mean a serious limitation. In cases on certain problems in the acoustics of halls, summary like this it has been found that good results may be in English), Ingeniiiren, no. 47 (1954 Nov.). obtained by using an M-S microphone as a "general" [2] H. Lauridsen, "Nogle Fors6g med in stereofonisk microphone, supplementing this with monaural accent System" (Some experiments on a system of stereophony, microphones, dividing the output of these into M and summary in English), lngenifren, no. 49 (1954 Dec.). S signals which are added in correct phase relation to [3] H. Lauridsen, "Das M,S Stereofonieverfahren the output of the general microphone [8], [9]. It is even und seine Anwendung zur Untersuchung raumakus- possible to adjust the M and S components in such a tischer Probleme" (The M-S system of stereophony way that the virtual sound sources at the receiver have and its application to the study of the acoustics of halls), a different spatial distribution from the real sound presented at the Fourth Congress of Programme En- sources. This effect might for instance prove useful in gineers at Detmold (1957 Oct.) and published by the dramatic productions. Nordwestdeutsche Musik-Akademie of Detmold (Ber- Artificial reverberation has also become a useful icht fiber die 4. Tonmeister-Tagung). feature of many types of programmes. It will be obvious [4] H. Lauridsen and F. Schlegel, "Stereofonie und from the effects described at the beginning of this paper richtungsdiffuse Klangwiedergabe" (Stereophony and that the S channel will be specially well adapted for omnidirectional sound reproduction), Gravesaner this, because a strong feeling of "roominess" may be Bl_itter, no. 5 (1956 Aug.). obtained while retaining clarity of reproduction. One [5] D. Kleis, "Experimente zur Verbesserung der method that seems promising is to add reverberation Raumwirkung yon Schall" (Experiments in improving to both M and S channels, but from different micro- the effect of space in sound reproduction), Elektronische phones in the echo room or different pickups on the Rundschau, no. 2, 64 (1955). reverberation plate, delaying the input to the S channel. [6] R. Vermeulen, "Stereo Reverberation" (Ster- An interesting device was developed by Mr. Lauridsen eophonic reverberation), Philips Technical Review, no. in connection with his work on stereophony: the acoustic 9, p. 258 (1956). goniometer [3]. In its fundamental form, this consists [7] F. Bergtold, "Ein neues Stereo-Mikrofon fQr of a combination of two velocity microphones placed Intensit/itsstereofonie" (A novel stereophonic mi- close together with their axes at right angles. The outputs crophone for intensity-stereophony), Funktechnik are connected to the X and Y plates of a cathode-ray (1957 Aug. 1). oscilloscope. The direction to a sound source may thus [8] F.-K. Schr6der, "Die sterephone SchallQber- be determined. If a sound source is moved in a circle tragung" (The stereophonic transmission of sound), around the microphones a rotating line of practically Funkschau, (1957 May 1). constant length will be displayed on the screen. [9] K. Bertram, "Aufnahmetechnik fQr kompatible Here again use was made of capacity microphones Stereofonie" (Studio technique for compatible stere- fitted with two diaphragms, one on each side of a per- ophony), Radio Mentor (1958 Sept.). forated central electrode. If all four diaphragms are [10] Clark, Dutton & Vanderlyn, "The"Stereosonic" connected separately to individual amplifying chains, Recording and Reproducing System," Proc. IEE (1957 several interesting combinations become possible. A Sept.). voltage proportional to the sound pressure, and thus [11] British Patent 394.325, Blumlein. omnidirectional, may be obtained. This may be eom- [12] British Patent 429.054, Blumlein. bined with the directional outputs to obviate the 180° [13] French Patent 860.672, Le Matfriel Tflfpho- uncertainty, as in radio-goniometry. Alternatively, the nique S.A. sound-pressure output may be used to modulate the [14] German Patent 1.010.569.

J. Audio Eng. Soc., Vol. 40, No. 10, 1992 October 807 HEEGAARD PAPERS-

Holger Lauridsen was bo m in 1920 in Esbjerg, Den, mark. He earned an M.S? degree in electrical engi- neering from The Technical University of Denmark in I944 and joined the Danish Broadcasting Corporation as Engineer in Charge in the Development Laboratory_ He redesigned the lacquer disc recording system then in use, but soon turned to audio tape recording. In t945 he designed a prototype portable recorder, which became the basis of a commercial product. His refined measurement techniques played a major rote in the standardization of record and playback response curves in the European Broadcasting Uniom Deeply interested in the _ubjectively assessed quali_ of broadcast music, Mr. Lauridsen then turned to acoustics, designed tape-based artificial reverberation units, dual 90-degree loudspeakers with time-compen- sated signals, and finally in I954 the compatible M/S Stereo System. He published very little about bis work, always feeling the need for a few more experiments to L provethevalidityofhis ideas_Hediedin themidstof H. Lauridsen this work in 1956, due to a heart attack,

Frederick D. Heegaard was born in 1898 in Copen- hagen, Denmark. He earned an M.S_ degree in electrical engineering from The Technical University of Denmark in 1926 and joined the Danish Post and Telegraph Administration. Broadcasting had been monopolized by theS_ate one year earlier, and the Engineering S er_ vices had been placed in the Post and Telegraph Administration. Mr, Heegaard dedicated his whole professional career to this new medium and became Engineer in Charge of Broadcast Services in I937. He was in charge of engineering planning and installation of Denmark'S first permanent Broadcasting Center Ra- diohuset from 1936 to 1941. From ! 953 to 1968 he was chairman of the European Broadcasting Union's Technical Working Party G, which produced the EB U Recommendation for ¼-inch ·audio tape, later to be confirmed by the CCIR, · In 1950 Engineering was transferred from Post and Telegraph to the Danish Broadcasting Corporation and Mr. Heegaard became Head of Engineering Services until 1964, when television had come of age, and he was made Director of Engineering, responsible for Sound and Television Engineering and Operational i Services.Heretiredin 1968, F.D. Heegaard

808 J.AudioEng.Soc.,Vol.40,No. 10,1992October